Nonlinearity in a channel is an inherent damage in an optical transmission system and is resulted from the Kerr Effect of the optical fiber. The effect of nonlinearity in the channel on the system includes: timing jitter, signal amplitude fluctuation and generation of ghost pulse, etc. [1].
When the rate of a single channel reaches 40-60 Gbits/s or higher, due to the effect of the dispersion, the pulses within the same channel will be greatly widened and overlap each other, with an energy exchange occurring between the overlapped pulses under the effect of the nonlinearity. In such a case, even though the remaining dispersion within the link is compensated at the receiving side, the system will still be subjected to severe nonlinear damage. And along with the increase in the capacity of the optical fiber transmission system, simple intensity modulation formats will be gradually replaced by more complex multidimensional modulation technologies. A higher incident power is often required in order to ensure that a complex modulation format has a sufficient signal to noise ratio, resulting in the increase in the nonlinear penalty of the system.
Currently, for a long-haul optical communication system, such schemes as link design, digital signal processing (DSP) of a receiver and encoding of transmitted signal, etc. are adopted to compensate or weaken the effect of the nonlinearity in the channel on the system. Following is a brief description of these schemes.
1) Scheme of Link Design:
The nonlinearity in the channel is weakened by adding an optical phase conjugate unit in the link, and in this way, the transmission haul may be increased to 6400 km from 5200 km [2].
2) Scheme of digital signal processing of a receiver, including a processing based on the channel inversion and a processing based on a nonlinear electrical filter, and wherein,
the processing based on the channel inversion: using the coherent receiver to obtain the electrical field signal containing an amplitude and phase of the signal light after being transmitted via a link, and constructing a virtual link completely opposite to the link parameters (dispersion coefficient, nonlinear coefficient and attenuation coefficient) according to the link configuration; and then passing the received electrical field signal through the virtual link, thereby compensating the damage of nonlinearity of the signal;
the processing based on a nonlinear electrical filter [5]: using a nonlinear electrical filter to equalize the signal being deteriorated by the nonlinearity.
3) Scheme of pre-compensating or encoding the transmitted signal at a transmitter side, comprising a pre-compensating scheme based on the channel inversion [7], a encoding scheme [8] and a new modulation scheme, wherein
Pre-compensating scheme based on the channel inversion [7]: firstly obtaining through calculation a signal of a transmission sequence with a certain length (a combination of various bits) after passing through a virtual link, and then transmitting a corresponding pre-compensating waveform according to the information sequence through a lookup table;
Encoding scheme [8]: reducing the times of occurrence of the worst bit sequence under the effect of the nonlinearity within the channel by increasing redundance;
New modulation scheme [9]: as to the OOK amplitude modulation signal, reducing effectively the effect of the four-wave mixing within the channel according to the phase of each of the symbols of the bit sequence.
However, it is found by the applicant in the implementation of the present invention that following defects exist in the above prior art:
1) the scheme of link design is not applicable to a laid optical fiber link and at the same time, there is no mature commercial module corresponding to the optical phase conjugate technology;
2) if the receiver adopts the scheme based on the channel inversion, it needs to solve the nonlinear Schrodinger equation by generally using a steply Fourier algorithm, and the system complexity depends on the size of the step to a certain extent; when the step is less than ⅓ of the span of the optical fiber, the performance of the compensation is optimal and thus, the complexity of calculation is excessively high, and even though the step is equal to the length of the span of the optical fiber, the number of multiplication needed in such scheme is more than 100 times greater than the linear filters needed to compensate the dispersion in the compensating link, which is a great challenge to the existing DSP technology [3][4];
3) the scheme in which the receiver side is based on nonlinear filters: in comparison with the scheme based on the channel inversion, the calculation complexity of this scheme is not reduced and even higher; and at the same time, since the processed signal is the signal after distortion (nonlinearity, frequency difference, and laser linewidth), it is somewhat difficult in algorithm convergence;
4) the pre-compensating scheme in which the transmitter side is based on the channel inversion: when the memory length of the system is relatively large, such as in a wireless dispersion compensating system, the number of the interactive symbols will reach 100 orders, and at this time, the size of the lookup table is 2100 or 4100 (dependent on the modulation scheme), which is hard to be realized;
5) the scheme of encoding at the transmitter side: the information rate is compromised.
Some documentations advantageous to the understanding of the present invention and the prior art are listed below, which are incorporated herein by reference, as they are stated fully in this text.                [1] IEEE PTL Vol. 12, No. 4, 2000, Antonio Mecozzi et. al.;        [2] OFC2004, PDP32, Chowdhury;        [3] Journal of Lightwave Technology, 2008, Kahn et. al.;        [4] IEEE Photonics Journal Volume 1, Number 2, August 2009; F. Yaman et. al.;        [5] JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 25, NO. 4, APRIL 2007, Chunmin Xia;        [6] ECOC, 2009, Yan Gao;        [7] IEEE Photonics Technology Lett., Vol. 18, 2006, pp. 403-405, K. Roberts et. al.;        [8] Journal Of Lightwave Technology, 2006, Vladimir Pechenkin et. al.;        [9] IEEE Photonics Technology Letters 2007, Ivan B. Djordjevic;        [10] Electrical domain compensation of non-linear effects in an optical communications system, CN1795627A, published on Jun. 28, 2006;        [11] Suppression of the Nonlinear Kerr Effect in Optical Fiber Communication Systems by Dispersion Management and Optical Phase Conjugation, a doctorial dissertation of Tsinghua University, November, 2006.        